1. Field of the Invention
The present invention relates to monitoring toner density of an unfused image in an electrophotographic machine, and, more particularly, to a toner patch sensor arrangement for monitoring toner density of an unfused image in an electrophotographic machine.
2. Description of the Related Art
Toner patch sensors are used in printers and copiers to monitor the toner density of unfused images and provide a means of controlling the print darkness. In color printers and copiers, the toner patch sensors are used to maintain the color balance and in some cases to modify the gamma correction or halftone linearization as the electrophotographic process changes with the environment and aging effects. It is a known problem that conventional reflection-based toner patch sensors will lose their calibration if the toner bearing surface changes in how the light is absorbed and scattered due to wear or toner filming.
Conventional reflection-based toner sensors use a single light source to illuminate a test patch of toner. In most cases the density of the toner patches are sensed on the photoconductor. With the advent of color laser printers with intermediate transfer belts, it is known to sense toner patches on the intermediate transfer medium rather than on the photoconductor surfaces. Toner patch sensing on the four photoconductor drums can be an unattractive option since it requires four sensors, and there may be no room for four such sensors between the cartridge and the intermediate belt.
It is known to use reflection signal ratios as opposed to differences in the toner patch signals. In a ratio control system, the reflectivity of a toner-free surface is sensed and compared to the reflectivity of the toned patch. By taking the ratio of these two signals, signal variations due to the variations in the light source, the detector, and the relative positions of these elements cancel out. However, this method of image density control is not self-compensating for degradation of the toner bearing surface, such as the photoconductive drum or intermediate belt, due to wear or toner filming.
Similar methods of maintaining accurate density control include sensing special toner patches with xe2x80x9csaturatedxe2x80x9d toner densities. Saturated patches on an intermediate surface can be sensed, and the resulting values can be used for density control and gradation correction.
Intermediate belts are prone to toner filming and mechanical wear. Since changes in the surface roughness of the intermediate belt will affect the amount of light that is scattered at the belt surface and the direction in which the light is scattered, the toner patch sensor needs to be made insensitive to the surface roughness of the intermediate belt surface.
What is needed in the art is a toner patch sensor arrangement that can accurately measure the toner thickness on a surface having various degrees of surface roughness.
The present invention provides a method of maintaining accurate density control independent of the intermediate belt surface roughness.
The invention comprises, in one form thereof, a toner patch sensor arrangement in an electrophotographic machine. A substantially hollow chamber has a reflective interior surface, a first opening exposing a toner patch, a second opening and a third opening. A light emitting element emits light onto the toner patch through the first opening and the second opening. A light detecting element receives through the third opening light reflected off of the toner patch such that at least a majority of the reflected light is received only after the light has also reflected off the interior surface of the chamber.
An advantage of the present invention is that toner thickness can be accurately measured on a surface having various degrees of surface roughness.
Another advantage is that only one photosensitive device is needed.